A study of mammograms spanning 25 years and thousands of patients has come to a startling conclusion: mammograms appear to be useless, at best.

The New York Timesreports that the study, published on Tuesday in the British Medical Journal, has shaken the medical community in part because it’s one of the most thorough studies of the procedure to date.

Researchers tracked more than 89,835 Canadian women, half of whom were randomly assigned to mammogram screenings. The other half had no mammograms and performed breast exams on themselves.

Twenty-five years later, the researchers found an identical rate of breast-cancer deaths in both groups. The mammograms’ only discernible impact was to elevate the diagnosis rate by 22%, or “overdiagnose” patients who were suffering from nonlethal forms of cancer and therefore could have avoided surgeries, chemotherapy and radiation treatments.

It’s not the first study to highlight the elusive impact of mammograms, nor is it the final word in this notoriously hard-to-measure field, but the New York Timesreports that the American Cancer Society will include it, along with every other study on mammography, in an official review of its guidelines this year. Whether it can nudge the advice of medical experts depends on how much weight they put on the polarizing new findings. For now, the consensus across every country’s medical guidelines, save Switzerland’s, is to keep screening.